As used herein, the term "sewage" refers not only to raw sewage, but also to by-products obtained during the treatment of the sewage. Similarly, the term "scum" as used herein means the portion of the sewage which floats on top of the raw sewage and which is conventionally skimmed from the top of the sewage. Sewage scum is primarily composed of fats, oils, greases, and the like, hereinafter sometimes collectively referred to as oil or as component product, and are mixed with both water and assorted solids. Due to the water and assorted solids in this sewage scum, it has a low combustibility. The unprocessed scum is contaminated with many undesirable elements. The scum contains dirt, rocks, sticks, human hair, and other floatables.
Sewage scum has previously been disposed of by grinding, landfill disposal, incineration or other methods. Such disposal of scum is not only expensive, but also a waste of a valuable energy source. Therefore, converting the scum to a usable oil and utilizing this valuable energy source eliminates cost associated with disposing the scum.
Because the scum has a very large water content and a very large solid content, there is a substantial variation of BTU in the product. This causes high maintenance problems with incinerators and problems with air pollution control, if the mixture is blended with sludge and then subsequently burned. The burning generates "hot spots" which increases maintenance cost. These "hot spots" are caused by the water and the solids found in the scum. Consequently, removal of solids is an additional problem. Grease or scum, which is fatty acid-based, has the ability to suspend a great many solids. Very fine solids are particularly difficult to remove.
According to prior art methods, emulsions are formed. Some portion of the water can be removed from them; however, a portion of the water is transformed to emulsions in known methods. These emulsions are generated when water and dirt particles are brought in close proximity by mixing, heating, or other physical-chemical processes used in the prior art. Heated air is a typical medium for emulsion generation. Steam coils traditionally used in heating waste fuels, generate emulsions by supplying energy so that heat, water, and oil are emulsified or forced into a phase which is neither a true oil phase or a water phase at the steam coils.
An addition problem is hair removal. Without efficient hair removal, subsequent treatment of the scum is rendered ineffective and the resulting product is undesirable.
Applicant has discovered that if the scum does not have substantial solids and water removed, variation in BTU content causes air pollution problems from both maintenance of equipment and air emulsions.
More particularly, U.S. Pat. No. 4,687,584 to Urbani discloses oil which is collected in a tank comprising oil skimmers, floating heads, and pumps. However, the Urbani process teaches heating scum with a heat exchanger, which aggravates scum problems. Specifically, this process would generate a great amount of emulsions. Similarly U.S. Pat. No. 4,775,388 to Beshore discloses heating sludge using a steam-containing atmosphere and maintaining a pressure of 10 to 700 psig. The temperature range of 90.degree. to 120.degree. C. or use of filtering or press operations dries the sludge The method for dewatering is not critical. This is in contrast to heating scum. Beshore further teaches odor control accomplished by an afterburner. U.S. Pat. No. 3,991,689 to Rinecker grinds the scum and provides a special nozzle which incinerates the product because the water and oil remain mixed The product has a variation in BTU values.
U.S. Pat. No. 4,248,709 to Irving discloses raw scum being introduced into an angled rotating drum. Heated air is claimed to separate oil from scum. The heated air generates substantial emulsions due to greater volumes of heated air used in this process, which is not desirable to effect separation. Municipal sewage treatment facilities decant the water from the scum, blend it with sewage sludge and burn or landfill the mixture Due to the impurities of this mixture, the burning mixture has two wide swings in BTU content which creates erratic "hot spots" and cooling periods.
U.S. Pat. No. 4,378,229 relates to a method for treating sewage in which the combustible components of the sewage are separated from the sewage and used as a fuel. Scum is collected from screened sewage. The scum comprises oils, greases, fats, water and intermixed solid material. Separation occurs by sedimentation so that the combustible materials are separated from other components. Then the combustible materials are separated from water and solid material by biochemical and biophysical reactions which provide for phase separation. Further dewatering and debris removal is effected by conventional means such as pumping. The product is combustible and can be used as a fuel of much lower BTU content than that of treated scum.
U.S. Pat. No. 1,586,449 to Jones discloses a centrifuge which separates water and emulsion from oil. This patent is based on petroleum-based oils and water rather than scum. U S Pat. No. 4,394,135 to Andress discloses a liquid hydrocarbon fuel containing stearic acid and linoleic acid. Scum is animal and vegetable-based oil with minor amounts of mineral oil.
Applicant has overcome the disadvantages associated with the prior art processes by recognizing the problems associated therewith, including emulsion formation, and by achieving a usable product substantially devoid of water, dirt and debris including human hair and having a low sulfur content. The product has a substantial uniform BTU content equivalent to #6 diesel fuel.